Many designs and types of fluid heaters for producing hot fluids, in particular hot water or steam, are known from the prior art. Household appliances in which fluid heaters are used include steam cleaners, steam irons, as well as appliances for preparing meals and/or hot beverages, such as coffee machines, espresso machines, milk frothers, steam cookers and others.
What matters to some degree at least for such appliances is that a fluid heater which uses electric energy to generate a hot fluid or steam from a cold fluid be integrated in what is already a compact device, i.e. one that has limited interior space. The fluid conduits must also withstand pressures of several atmospheres, particularly when steam is produced.
EP 1 321 708 shows a fluid heater for making hot water or steam, comprising a fluid conduit and a heating device, the heating device consisting of at least two tubular heating elements connected in a heat-conducting manner to the fluid-conducting pipe. In order to increase the area of contact between the heating device and the fluid-conducting pipe, the fluid-conducting pipe and the tubular heating element are provided with matching contact surfaces such that either the tubular heating elements have substantially convex contact surfaces and the fluid-conducting pipe has substantially concave contact surfaces to match, or both the tubular heating element and the fluid-conducting pipe have substantially planar contact surfaces. Like many other devices known from the prior art, it is also proposed that the components consisting of a fluid-conducting pipe and a heating device be produced in the form of an extrusion profile, with the intended geometry of contact surfaces being formed between the fluid-conducting pipe and the tubular heating elements.
A common feature of prior art devices, with regard to obtaining high heating capacities in a confined space, is that a maximum proportion of the fluid guiding pipe is in good heat-conducting contact with the heating device. Until now, round cross-sections for the fluid conduits guiding core have given rise to the problem that the ratio between the heated interface and the fluid volume in the fluid conduit is too large and that a cold core flow ensues. This requires relatively long fluid conduits for inputting the thermal energy needed for the desired fluid temperature. In the case of the prior art discussed above, the fluid guiding pipe, which has a circular cross-section in its original state, is deformed during manufacture in order to reduce the cross-section that is heated during operation. This requires a respective deformation step during manufacture.